i965/vs: Make vec4_vs_visitor class derived from vec4_visitor.

This patch just creates the derived class; later patches will migrate
VS-specific functions and data structures from the base class into the
derived class.
!UPSTREAM ===== BEGIN rebase of calim's GS patches =====
draw/gs: fix allocation of buffer for GS output vertices

Before, it accounted for the size of the vertices but not the other fields
in the vertex_header struct, which caused memory corruption.
mesa: add EXT_geometry_shader4 and ARB_geometry_shader4 to extension list
mesa: account for geometry shader texture fetches in update_texture_state
glcpp: define extension macros for EXT/ARB_geometry_shader4
glsl: add builtins for EXT/ARB_geometry_shader4
glsl: add ir_emitvertex and ir_endprim instruction types

These correspond to the EmitVertex and EndPrimitive functions in GLSL.

v2 (Paul Berry <stereotype441@gmail.com>): Add stub implementations of
new pure visitor functions to i965's vec4_visitor and fs_visitor
classes.
glsl: lower gl_ClipDistanceIn to gl_ClipDistanceInMESA in lower_clip_distance

This corresponds to the lowering of gl_ClipDistance to gl_ClipDistanceMESA.
It converts the 2D array gl_ClipDistanceIn from EXT/ARB_geometry_shader4 to
a 1D array of vec4s, gl_ClipDistanceInMESA.
glsl: support compilation of geometry shaders via EXT/ARB_geometry_shader4

This commit adds all of the parsing and semantics for EXT/ARB_geometry_shader4,
including support for 2D arrays as inputs.

v2 (Paul Berry <stereotype441@gmail.com>): Adjust i965's
brw_link_shader() to pass the new is_geometry_shader argument to
do_set_program_inouts.  Add a few missing calls to
get_pipeline_stage().  Fix some signed/unsigned comparison warnings.
Fix handling of NULL consumer in assign_varying_locations().
mesa: Refactor copying of linked program data.

This patch creates a single function to copy the the UsesClipDistance
flag from gl_shader_program.Vert to gl_vertex_program.  Previously
this logic was duplicated in the i965-specific function
brw_link_shader() and the core mesa function _mesa_ir_link_shader().

This logic will have to be expanded to support geometry shaders, and I
don't want to have to update it in two separate places.
mesa: Copy linked program data for GS.

The documentation for gl_shader_program.Geom and gl_geometry_program
says that the former is copied to the latter at link time, but this
wasn't happening.  This patch causes _mesa_ir_link_shader() to perform
the copy, and updates comment accordingly.
st/mesa, glsl_to_tgsi: add support for geometry shaders

Note (Paul Berry <stereotype441@gmail.com>): I'm not sure I rebased
this correctly.  It appears that some amount of 2D array support was
added to tgsi after the patch was originally written.
WIP: extract virtual functions from ir_variable visitor

Includes making a new function to handle variables of type
ir_var_system_value, which has different meanings in VS and GS.

The following functions have been renamed to reflect their generic
purpose, rather than their VS-specific behaviour:

- emit_attribute_fixups() -> emit_prolog()
- emit_vertex_program_code() -> emit_program_code()
- emit_urb_writes() -> emit_thread_end()
WIP: move VS-specific functions to vec4_vs_visitor
WIP: move VS-specific data members to vs_vec4_visitor.

This required making some new data members in the base class to point
to generic data structures.
WIP: Generalize data structures pointed to by vec4_generator.

vec4_generator never used the vertex-shader-specific parts of these
data structures anyway.
WIP: rename vec4_generator::generate_vs_instruction
WIP: make stuff safe to include from C++
i965: Stop including brw_vs.h from brw_vec4.h

This is backwards from what we are going to want in the long term, which is:

- brw_vec4.h declares general-purpose vec4 infrastructure needed by
  both VS and GS
- brw_vs.h includes brw_vec4.h and adds VS-specific parts.
- brw_gs.h includes brw_vec4.h and adds GS-specific parts.

Note that at the moment brw_vec.h contains a fair amount of
VS-specific declarations--I plan to address that in a later patch.